Is it possible to make visible spotlight lightrays?
Yes, this is now very easy to do efficiently. Add a spot lamp where you want it, and then add a cube and edit to just barely contain the spot lamp:



Make the material of the cube as shown. Note that the Transparent shader is not necessary, and you can leave the Surface input blank. However you can have a surface material and a volume material at the same time.



Then add a plane as the ground and render:



Since the volume is so small to render, it renders very quickly, this test render took about 3.5 seconds.

If you want to move your spot lamp, just parent the mesh to it with CtrlP > Object.